I exported a "Pea-Happy.png" and looked at it in Paint.NET, and I noticed that there are some pixels that are right on the border of the PNG image (they are semi-transparent, and blended). I'm guessing these are the culprit.

So I think if you create a slightly bigger, blank PNG (maybe 40x40 pixels) and then paste in the pea (37x37 pixels), then the boundary detection will be happy...